The invention relates to an engine braking method for an internal combustion engine having two serially arranged exhaust-gas turbochargers, each having an exhaust gas turbine with a bypass line including a valve for controlling the exhaust gas flow through the turbines.
DE 198 53 360 A1 discloses an internal combustion engine which is equipped with two exhaust-gas turbochargers which are connected in series. The larger of the two superchargers is arranged remotely from the engine and performs the function of the main supercharger, and the smaller is arranged near the engine and functions as an additional supercharger which can be placed in service as necessary. The connection and disconnection of the additional supercharger is carried out using a shut-off device which comprises a rotary slide valve in the intake manifold and also one in the exhaust manifold. When the additional supercharger is connected the mass flows in the intake manifold and in the exhaust manifold are directed through the compressor or, respectively, the turbine of the additional supercharger and, in contrast, when the additional supercharger is disconnected the mass flows are directed to bypass the additional charger via bypass lines. Furthermore, a bypass is provided for bypassing the exhaust gas turbine of the main supercharger.
The two-stage supercharging can be used to increase the power both in the powered engine operation and in the engine braking mode. In order to achieve a high engine braking power, a variable turbine geometry in the exhaust gas turbine of the main supercharger is moved to the shut-off position as a result of which an increased exhaust gas back pressure is generated between the outlet of the internal combustion engine and the inlet of the turbine with a variable turbine geometry. The exhaust gas flows via the remaining open flow cross section of the variable turbine geometry into the turbine and strikes the turbine wheel at high flow speed, so that the compressor wheel is also driven at high speed and an increased supercharging pressure is generated in the intake manifold. In this way, an increased pressure level is generated in the cylinders, against which the pistons of the internal combustion engine have to perform compression work on the air side and also on the exhaust gas side. The exhaust gas back pressure and the supercharging pressure can be increased further by the connection of the additional supercharger.
Based on this prior art, it is the object of the pre-sent invention to provide an arrangement for varying the engine braking power in an internal combustion engine with two serially arranged exhaust-gas turbochargers over a wide power spectrum by using only simple means. More specifically, it is the object of the invention to provide both, for a high engine braking power and the possibility of an adjustment to a desired target value, for example for providing a cruise control function.